The overall objective of this research program is to examine the effects of diet and environmental factors (such as photoperiod) on energy balance and adiposity in Syrian hamsters (Mesocricetus auratus). Hamsters fed a high-fat diet are unusual among animal models of dietary obesity. They do not overeat, but they become obese because of decreases in energy expenditure. This decrease in actual energy expenditure is accompanied by increases in thermogenic capacity and brown adipose tissue mass, protein content, and DNA content. All of these effects of high-fat diets (e.g., increases in body weight, metabolic efficiency, adiposity, thermogenic capacity, and brown adipose tissue size) are exaggerated by short photoperiods or treatment with melatonin. These diet- and photoperiod-induced changes in metabolic efficiency and thermogenic capacity may be seasonal responses which are beneficial in preparing Syrian hamsters for winter. One group of experiments will further characterize the high-fat-induced obesity and will examine the effects of voluntary exercise and cold exposure. A second group of experiments will focus on the effects of photoperiod and melatonin on energy balance and brown adipose tissue in fat-fed hamsters. The third group of experiments will investigate potential mechanisms mediating the effects of short photoperiods/melatonin including sympathetic innervation of brown adipose tissue, melatonin action in the brain, and gonadal and thyroid hormones. This multidisciplinary approach to the study of obesity (borrowing concepts and techniques from endocrinology, neurobiology, and nutrition) should yield new insights into how environmental factors such as diet quality, photoperiod, and temperature can interact to influence energy metabolism and body composition. Finally, fat-fed hamsters, which become quite obese without overeating, may be a useful animal model of some forms of human obesity.